To meet the different requirements, the most diverse solutions have been developed. Equipment with a higher demand is supplied with energy stored in an accumulator. In case of necessity the voltage is stabilized or transformed to the form needed. This solution is, however, too expensive, in particular, when a high output is required, and when a long-lasting power cut with an infrequent occurrence is to be met in such a manner. The costs of said equipment are further increased by the fact that a continuous maintenance is imperative, at the same time--in spite of the most careful maintenance--the accumulators deteriorate in a few years.
In case of less strict requirements, when a drop-out in the supply from the mains occurs, the operator puts the Diesel-motor of the auxiliary electrical source into operation and in such a manner energy supply may be ensured within some minutes. With this solution there is a considerable break however, the duration of which depends on the reliability and skill of the operator.
In recent years devices have been developed, where the electromotor supplied from the mains performs a steady rotary motion and simultaneously drives the flywheel. In case of a drop-out the engine functions as a generator and supplies energy to the equipment having been separated from the mains, at the same time by the quick locking of a frictional clutch it is connected to the system, whereby the stand-by Diesel-motor is started. According to the relevant publications the Diesel-motor reaches the nominal speed of rotation within a few hundredth seconds and takes over the drive of the generator from the flywheel. The disadvantage of this solution lies in that when locking the clutch, utmost high mechanical stresses arise, accordingly, the equipment endures such "quick starts" to a limited extent only.